1. Field of the Invention
The invention relates to a process to induce polymerization of an organic electronically conductive polymer in the presence of a partially delithiated alkali metal phosphate which acts as the polymerization initiator.
2. Description of the Related Art
Lithium-ion batteries have known a phenomenal technical success and commercial growth since the initial work by Sony in the early 90's based on lithium insertion electrodes, essentially the high voltage cobalt oxide cathode invented by J. B. Goodenough and the carbon anode using coke or graphitized carbonaceous materials.
In the mid 90's, Goodenough (See U.S. Pat. Nos. 5,910,382 and 6,391,493) suggested that polyanionic phosphate structures, namely nasicons and olivines, could raise the redox potential of low cost and environmentally compatible transition metals such as Fe, until then associated to a low voltage of insertion. For example LiFePO4 was shown to reversibly insert-deinsert lithium-ion at a voltage of 3.45 V vs a lithium anode corresponding to a two-phase reaction. Furthermore, covalently bounded oxygen atom in the phosphate polyanion eliminates the cathode instability observed in fully charged layered oxides, making an inherently safe lithium-ion battery.
As pointed out by Goodenough (U.S. Pat. Nos. 5,910,382 & 6,514,640), one drawback associated with the covalently bonded polyanions in LiFePO4 cathode materials is the low electronic conductivity and limited Li+ diffusivity in the material. Reducing LiFePO4 particles to the nanoscale level was pointed out as one solution to these problems as was the partial supplementation of the iron metal or phosphate polyanions by other metal or anions.
One significant improvement to the problem of low electronic conductivity of complex metal oxide cathode powder and more specifically of metal phosphate was achieved with the use of an organic carbon precursor that is pyrolysed onto the cathode material or its precursor to improve electrical field at the level of the cathode particles [Ravet (U.S. Pat. Nos. 6,963,666, 6,855,273, WO 02/027824 and WO 02/027823)].
Preparation of a composite cathode of complex metal oxide with an electronically conductive polymer (ECP) could also overcome low electronic conductivity of complex metal oxide, as demonstrated for example by Wang et al. [An investigation of polypyrrole-LiFePO4 composite cathode materials for lithium-ion batteries, Electrochimica Acta, 50 (2005) 4649-4654]. Wang disclosed preparation of a LiFePO4-polypyrrole composite cathode by chemically initiating polymerization of pyrrole by FeCl3 in a water dispersion of LiFePO4 and sodium p-toluene-sulfonate as counter-anion. Goodenough et al. also disclosed in WO 06/130766 composite of pyrolytic carbon-coated LiFePO4 (C—LiFePO4) and polypyrrole obtained by electropolymerization of pyrrole.
At industrial scale, electropolymerization is not a convenient process and known chemical routes to prepare LiFePO4-ECP composite are also unsatisfactory.
Problems remain to find a convenient and up-scalable process allowing preparation of surface modified lithium metal phosphate with an electronically conductive polymer.